Tropical Cyclones & Tornadoes

1. Tropical Cyclones&Tornadoes

2. Formation of Tropical Cyclones • Warm ocean waters (> 26.5°C) throughout a sufficient depth (> 50 m) • An atmosphere is potentially unstable to moist convection • Relatively moist layers near the mid-troposphere (~5 km) • A minimum distance of at least 500 km from the equator for non-negligible amounts of the Coriolis force • A pre-existing near-surface disturbance with sufficient vorticity and convergence • Low values (< 10 m/s) of vertical wind shear between the surface and the upper troposphere

3. Air Flowing into a Low Pressure Area(North Hemisphere)

4. Locations of Tropical Cyclone Formation for a 20-year Period

5. Nature of Tropical Cyclones • Huge whirlpools in atmosphere • A disc-like shape with a vertical scale of tens of kilometres against horizontal dimensions of hundreds of kilometres • The rainbands rotate in the same sense as the storm circulation (anti-clockwise in the Northern Hemisphere but clockwise in the Southern Hemisphere - Coriolis force) • A travelling heat engine (feeds on latent heat released from condensation in moist air) • The "eye" is typically of the order of tens of kilometres in diameterand relatively calm • Lifespan varying from a few days to a few weeks

6. A Tropical Cyclone in the Northern Hemisphere

7. A Tropical Cyclone in the Southern Hemisphere

8. Structure of Tropical Cyclones • Warm core system • Low level pressure is lowest at the centre (convergence) • High level pressure is highest at the centre (divergence) • The region of phenomenal winds is confined to a ring around the eye wall (conservation of angular momentum) • Air is ascending around the eye wall and descending at the centre

9. Model Vertical Profile of a Mature Typhoon

10. Wind Speed Record during the Passage of Hurricane Celia (1970)at Gregory, Texas, USA.

11. Three Dimensional Air Trajectories through a Typhoon as Simulated by the Computer.

14. Tracking and Locating • Satellite analysis (especially over the vast expanse of the oceans) • A well-formed eye is a definite indicator of where the storm is • Tracing the spiral rainbands of the storm or studying the relative motion of cloud features near its centre • Tracking on the radar (if the storm is located within 500 km of Hong Kong)

15. Typhoon York (颱風約克 - 1999) as Seen from the Radar

16. Infra-red (left) and Visible (right) Satellite Pictures of Tropical Storm Hope (1989) Taken at the Same Time

17. Examples of Factors that Need to be Considered in Track Forecasting • Inertial tendency (drift towards the northwest in the Northern Hemisphere) • The environmental current • Westward extent of the Pacific ridge • North-south migration of the ridge axis • Point of weakness in the ridge • Interaction with mid-latitude weather systems • Interaction with other tropical cyclone(s) • Interaction with terrain • Weak flow region where the steering current is ill-defined

18. Typical Tracks of Tropical Cyclones over the Western North Pacific

19. The Fujiwhara Effect（藤原效應） • A paper describing the motions of "vortices" in water by Dr. Sakuhei Fujiwhara in 1921 • Interactions between vortices • If two vortices are equal in size and strength and spin in the same direction => rotate around one another • If two vortices spinning counter-clockwise approach and one of the vortices is larger than the other => the smaller of the two vortices will get caught in the circulation of the larger one and be gobbled up eventually

20. Methods Used for Forecasting the Track of Tropical Cyclones • Conventional weather chart analysis • Climatology and statistics • Statistical-dynamical methods • Numerical models • Remark: The science of tropical cyclone forecasting is by no means perfect. The multitude of contributing factors are so complex that they are as yet not entirely understood, or are only crudely represented (sometimes for practical reasons) in the various forecast schemes.

21. A Numerical Weather Forecast Map (NGP)

22. Typhoon Season of Hong Kong in 1999

23. Power of Tropical Cyclones • Generating mechanical power of about 1.5 x 1012 W (~ half the world-wide electricalgenerating capacity) • Sustained winds on record: 165 kt (306 km/hr) • Minimum surface pressure on record: 870 mb • Height of surge on record: 13 m • Rainfall on record: 1,825 mm in 24 hrs. • Size on record: 1,100 km in radius • Life-span on record: 31 days • Casualties on record: 300,000 people dead

24. Typhoons Necessitating the Hoisting of the Hurricane Signal No.10

25. Typhoon Wanda (颱風溫黛 - 1962) • Duration: August 27 to September 2, 1962 • Maximum sustained winds (HKO): 72 knots [133km/h] (Record) • Maximum gust: 140 knots [259 km/h] (HKO); 153 knots [284 km/h] (Tate's Cairn) (Record) • Closest distance from HKO: 20 km SSW • Instantaneous minimum surface pressure (HKO): 953.2 mb (Record) • Daily amounts of rainfall (HKO): 203.0 mm (September 1) • Maximum tide height (Tai Po): 5.4 m • Casualties: 130 persons dead (Record)

26. Record of Wind Speed of Typhoon Wanda

27. Record of Surface Pressure of Typhoon Wanda

28. Damage due to Typhoon Wanda

29. Typhoon Wayne (颱風韋因 - 1986) • Duration: August 18 to September 6, 1986 • Tropical cyclone signals were hoisted and lowered on three separate occasions for the same storm • Classic example of delicate interplay and balance among various meteorological factors • The Fujiwhara effect

30. Typhoon Wayne as Seen from the Radar

31. Interaction between Wayne and Another Typhoon named Vera - the Fujiwhara Effect

32. Formation of Tornadoes • Spawned in thunderstorms • Vertical shear of horizontal wind （垂直風切變） • Horizontal vorticity formed (aligned with the average wind direction - streamwise vorticity) • Convection (updrafts) in thunderstorms distort the vortex tube from horizontal to vertical • Regions of rotating updraft air in the thunderstorms formed (mesocyclones) • The rotations in mesocyclones concentrated to form tornadoes (similar to the water vortices - vertical stretching)

34. Nature of Tornadoes • Vortices in atmosphere • Funnel shapes with diameters on the scale of 100s of meters (Coriolis force is negligible in such small system) • Rotate anti-clockwise in many cases (the North Hemisphere) • Low pressure centres (capricious effect may be caused) • Lifespan measured in minutes

35. Tornado Variations and Waterspouts（水龍捲） • Some tornadoes may form during the early stages of rapidly developing thunderstorms • Tornadoes may appear nearly transparent until dust and debris are picked up • Occasionally, two or more tornadoes may occur at the same time • Tornadoes (weak) that form over warm water are called waterspouts • Waterspouts occasionally move inland becoming tornadoes causing damage and injuries

36. Tornadoes Spawned by Tropical Cyclones (TC Tornadoes) • Spawn when certain instability and vertical shear criteria are met • The smaller and shallower storm cells => weaker tornadoes (no F5-rated TC tornadoes in the past 50 years of reliable data) • Almost all tropical cyclones making landfall in the United States spawn at least one tornado • The right-front quadrant of a tropical cyclone (relative to TC motion) is strongly favored for tornado formation (the Northern Hemisphere) • Tornadoes may be spawned up to three days after landfall of tropical cyclones • Hurricane Beulah spawned a reported 141 tornadoes in southeast Texas during the first several days after its landfall in September 1967 • TC tornadoes are especially difficult to deal with because the unusually small storm cells that may be overlooked

37. A Waterspout Occurred during Typhoon Durian （颱風榴槤 - 2001)

38. Power of Tornadoes • In an average year, 800 tornadoes are reported, resulting in 80 deaths and over 1,500 injuries in USA • Wind speeds can approach 800 km/hr. • Damage paths can be in excess of one mile wide and 50 miles long • Lifetime of violent tornadoes can exceed 1 hour